It is known that dilute sulfuric acid containing metal sulfates, in particular so called waste acid from the sulfate process for the production of titanium dioxide, can be concentrated by multi-stage vacuum evaporation until the metal sulfates become substantially insoluble and can be separated from a 62 to 70% sulfuric acid which is returned to the process (EP-A 133 505). This process is, however, accompanied by losses of sulfuric acid and metal sulfates leading to contamination of the vapor condensate.
These losses must be reduced in view of the increasingly more stringent standards applied to the quality of effluent.
Precipitation of calcium sulfate and metal hydroxides from the vapor condensate by the addition of calcium compounds results in highly diluted suspensions which are difficult to filter. The filter cake has to be deposited which additionally results in environmental problems.